Dielectric composition with reduced resistance

ABSTRACT

This invention provides a dielectric composition comprising a dielectric which is fireable in air at a temperature in the range of about 450° C. to about 550° C. and a conductive oxide selected from the group consisting of antimony-doped tin oxide, tin-doped indium oxide, a transition metal oxide which has mixed valence states or will form mixed valence states after firing in a nitrogen atmosphere at a temperature in the range of about 450° C. to about 550° C. and normally conducting precious metal oxides such as ruthenium dioxide, wherein the amount of conductive oxide present is from about 0.25 wt % to about 25 wt % of the total weight of dielectric and conductive oxide. This dielectric composition has reduced electrical resistance and is useful in electron field emission devices to eliminate charging of the dielectric in the vicinity of the electron emitter and the effect of static charge induced field emission.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Appl. No.60/291,408, filed on May 16, 2001.

FIELD OF THE INVENTION

This invention provides a dielectric composition with reduced electricalresistance that is useful in electron field emission devices toeliminate charging of the dielectric in the vicinity of the electronemitter and the effect of static charge induced field emission.

BACKGROUND OF THE INVENTION

In certain designs of cathode plates for field emission devices, e.g.,field emission displays, the layout of the cathode surface may requirethe emitter lines to be bordered by or in close proximity toelectrically insolating dielectric surfaces. During field emission,these neighboring dielectric surfaces are bombarded by high energyelectrons with shallow angle of incidence. As a result, secondaryelectron emission may occur from the dielectric surface leaving behindstatic positive charges. These positively charged surfaces, being only ashort distance away from the emitter, will exert a large positiveelectric field on the emitter resulting in even stronger emission in thedirection of the dielectric surfaces. This effect is thereforeself-enhancing and will dominate any gate or anode control. Since thestatic charge continues to build with time, uncontrollable charginginduced emission can occur even at very low continuous anode voltage.This problem severely limits the anode voltage. One solution would be tochange the cathode layout that may minimize this charging effect. Apreferred solution is to develop a single or multilayer dielectricsystem that does not charge up as a result of having a low secondaryemission characteristic or effectively dissipating the charge as theelectric field increases.

SUMMARY OF THE INVENTION

This invention provides a dielectric composition comprising a dielectricand a conductive oxide, wherein the dielectric is fireable in air at atemperature in the range of about 450° C. to about 550° C. and theconductive oxide is selected from the group consisting of antimony-dopedtin oxide, tin-doped indium oxide, a transition metal oxide which hasmixed valence states or will form mixed valence states after firing in anitrogen atmosphere at a temperature in the range of about 450° C. toabout 550° C. and conducting precious metal oxides such as rutheniumdioxide. The amount of conductive oxide present is from about 0.25 wt %to about 25 wt % of the total weight of dielectric and conductive oxideand preferably from about 0.5 wt % to about 15 wt %. The dielectriccomposition must be capable of being fired in nitrogen after first beingfired in air in order to be compatible with the treatment necessary informing an electron emitter cathode assembly.

This invention also provides a paste for screen printing a dielectriccomposition comprising a dielectric and a conductive oxide, wherein thedielectric is fireable in air at a temperature in the range of about450° C. to about 550° C. and the conductive oxide is selected from thegroup consisting of antimony-doped tin oxide, tin-doped indium oxide, atransition metal oxide which has mixed valence states or will form mixedvalence states after firing in a nitrogen atmosphere at a temperature inthe range of about 450° C. to about 550° C. and conducting preciousmetal oxides such as ruthenium dioxide. The amount of conductive oxidepresent is from about 0.25 wt % to about 25 wt % of the total weight ofdielectric and conductive oxide and preferably from about 0.5 wt % toabout 15 wt %.

Such dielectric compositions have reduced resistances and are useful infield emission devices.

There is also provided a preferred dielectric for use in the dielectriccomposition of the invention. This dielectric is a solid solutioncomprised of about 1 to about 26 wt % SiO₂, about 0.5 to about 6 wt %Al₂O₃, about 6 to about 24 w % B₂O₃, about 2 to about 24 wt % ZnO, about0.1 to about 5 wt % Na₂O and about 20 to about 75 wt % Bi₂O₃.Preferably, the solid solution is comprised of about 1 to about 9 wt %SiO₂, about 0.6 to about 6 wt % Al₂O₃, about 6 to about 14 wt % B₂O₃,about 2 to about 13 wt % ZnO, about 0.1 to about 2 wt % Na₂O and about65 to about 72 wt % Bi₂O₃. More preferably, the solid solution iscomprised of about 2 wt % SiO₂, about 3 wt % Al₂O₃, about 13 wt % B₂O₃,about 9 wt % ZnO, about 1 wt % Na₂O and about 72 wt % Bi₂O₃.

The preferred conductive oxide is antimony-doped tin oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of the electron emitter configuration usedin Examples 5-1 and Comparative Experiments A-C.

FIG. 2 shows the emission results obtained with the sample ofComparative Experiment A.

FIG. 3 shows the emission results obtained with the sample of Example 5.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a dielectric composition with reducedresistance to reduce or eliminate the charge buildup on the dielectricby introducing a small amount of semiconducting or metallic material. Atsome concentration of the semiconducting or metallic material, theresistance will drop to the range which is low enough to allow effectivestatic charge dissipation at high fields while still providing therequired insulating properties to prevent shorting between adjacentemitter lines or dots. Such a dielectric composition may serve as theentire dielectric or may also be used as a top layer in a multilayerdielectric system which provide good cross layer insulation andeffective surface charge dissipation.

The dielectric composition provided by this invention comprises adielectric which is fireable at a temperature in the range of about 450°C. to about 550° C. and a conductive oxide selected from the groupconsisting of antimony-doped tin oxide, tin-doped indium oxide, atransition metal oxide which has mixed valence states or will form mixedvalence states after firing in a nitrogen atmosphere at a temperature inthe range of about 450° C. to about 550° C. and normally conductingprecious metal oxides such as ruthenium dioxide. The amount ofconductive oxide present is from about 0.25 wt % to about 25 wt % of thetotal weight of dielectric and conductive oxide and preferably fromabout 0.5 wt % to about 15 wt %. The optimum amount of conductive oxiderequired to achieve the desired resistivity will vary depending upon theparticular conductor used and its state of dispersion. The desiredresistivity of the dielectric composition will depend on the particularconfiguration of the emitter, i.e., the size of the emitter lines ordots and the separation of the emitter lines or dots. The amount ofconductive oxide can be adjusted to the particular configuration used.

The paste for screen printing the dielectric composition of theinvention will typically contain the dielectric powder, the conductiveoxide, an organic medium, solvent, and surfactant. The role of themedium and solvent is to suspend and disperse the particulateconstituents, i.e., the solids, in the paste with a proper rheology fortypical patterning processes such as screen printing. There are a largenumber of such mediums known in the art. Examples of resins that can beused are cellulosic resins such as ethyl cellulose and alkyd resins ofvarious molecular weights. Butyl carbitol, butyl carbitol acetate,dibutyl carbitol, dibutyl phthalate and terpineol are examples of usefulsolvents. These and other solvents are formulated to obtain the desiredviscosity and volatility requirements. A surfactant can be used toimprove the dispersion of the particles. Organic acids such oleic andstearic acids and organic phosphates such as lecithin or Gafac®phosphates are typical surfactants. If the screen-printed paste is to bephotopatterned, the paste contains a photoinitiator, a developablebinder and a photohardenable monomer comprised, for example, of at leastone addition polymerizable ethylenically unsaturated compound having atleast one screen printing typically contains the dielectric frit and theconductive oxide, an organic medium, solvent and surfactant.

The frit used in pastes for screen printing dielectrics is typically anappropriate mixture of oxides. As is well known to those skilled in theart, inert fillers can be added to the frit to modify some of theproperties, e.g., flow characteristics of the frit, strength of thefired composition, temperature coefficient of expansion, etc. All suchpastes and the resulting fired dielectric compositions can contain 0-35wt % of inorganic filler. Typical examples of such filler are alumina,silica, calcium zirconate and zircon. For the purposes herein,dielectric means all of the dielectric including any filler present.

The preferred dielectric frit for use in pastes to screen print thedielectric compositions of the invention is comprised of about 1 toabout 26 wt % SiO₂, about 0.5 to about 6 wt % Al₂O₃, about 6 to about 24w %, B₂O₃, about 2 to about 24 wt % ZnO, about 0.1 to about 5 wt % Na₂Oand about 20 to about 75 wt % Bi₂O₃. More preferably, the dielectricfrit is comprised of about 1 to about 9 wt % SiO₂, about 0.6 to about 6wt % Al₂O₃, about 6 to about 14 w % B₂O₃, about 2 to about 13 wt % ZnO,about 0.1 to about 2 wt % Na₂O and about 65 to about 72 wt % Bi₂O₃. Mostpreferably, the dielectric frit is comprised of about 2 wt % SiO₂, about3 wt % Al₂O₃, about 13 wt % B₂O₃, about 9 wt % ZnO, about 1 wt % Na₂Oand about 72 wt % Bi₂O₃.

After firing, this paste provides a dielectric which s a solid solutionof SiO₂, Al₂O₃, B₂O₃, ZnO, Na₂O and Bi₂O₃ with the above ratio weightpercent.

EXAMPLES OF THE INVENTION Examples 1-4

The purpose of these Examples was to compare various conductive oxidesfor the dielectric composition.

Four pastes were prepared using a bismuth-based dielectric frit andCr₂O₃, V₂O₅, V₂O₃, and SnO₂:Sb. The first three oxides were standardpowder reagents. The antimony doped tin oxide, SnO₂:Sb, was Zelec® ECP3010-XC, which can be obtained from E. I. du Pont de Nemours andCompany, Wilmington, Del.

The bismuth-based frit used in these Examples, Bi-frit, has thecomposition shown in Table I:

TABLE I Ingredient Wt % Bi₂O₃ 71.8 B₂O₃ 13.2 ZnO 9.0 Al2O3 3.0 SiO2 2.0Na2O 1.0

The four pastes were prepared with the compositions shown in Table II:

TABLE II Example 1 Example 2 Example 3 Example 4 Ingredient Wt % Wt % Wt% Wt % Bi-frit 66.0 66.0 66.0 66.0 SnO₂:Sb 16.5 None None None Cr₂O₃None 16.5 None None V₂O₅ None None 16.5 None V₂O₃ None None None 16.5Vehicle 16.4 16.4 16.4 16.4 Surfactant  0.6  0.6  0.6  0.6 Pigment  0.5 0.5  0.5  0.5

The vehicle is a standard thick film paste ingredient consisting of amixture of 10% ethylcellulose in a beta terpineol solvent. Thesurfactant is an organophosphate, Gafac RE-610. The pigment is standardpigment grade cobalt aluminate.

Each of the four pastes were printed onto a glass slide in the form apad 0.25 inch (0.6 cm) by 1 inch (2.5 cm). Each pad was about 10 μmthick. Silver electrodes were screen printed onto each pad to measurethe resistance using a silver paste composition 7095 available from E.I. du Pont de Nemours and Company, Wilmington, Del. The gap between thesilver electrodes over which the resistance was measured was about 1 mm.The resistance of each sample was measured after firing in air at 525°C. and then after firing in nitrogen at 510° C. The electricalproperties of the four fired pastes are shown in Table III below:

TABLE III Air Fired Nitrogen Fired Example 1  1.2 MΩ 0.9 MΩ Example 2  >1 GΩ  >1 GΩ Example 3  540 MΩ   7 MΩ Example 4   15 MΩ  23 MΩ

All of the conductive oxides reduced the resistance of the dielectric.SnO₂:Sb showed the largest reduction after firing in air and the leastchange after undergoing the second firing in nitrogen.

Examples 5-11, Comparative Experiments A-C

These Examples and Comparative Experiments show the effects of variousamounts of the conductive oxide SnO₂:Sb in eliminating the charging ofthe dielectric and the resulting undesirable electron emission.

The paste of Example 1 was blended with a paste with the compositionshown in Table IV:

TABLE IV Ingredient Wt % Bi-frit 70.0 SnO₂:Sb None Vehicle 29.0Surfactant  0.5 Pigment  0.5where the Bi-frit, vehicle, surfactant and pigment are identical tothose used for the paste of Example 1. Ten different pastes were formedwith SnO₂:Sb content ranging from 0 to 10 wt % based on the total weightof the paste. The SnO₂:Sb content of each Example and ComparativeExperiment is shown in Table V below:

TABLE V Ingredient Example or wt % Comp. Exp. SnO₂:Sb ResistanceCharging A 0 >200 GΩ Extensive B 2 >200 GΩ Extensive C 3 ~200 GΩ Some  53.5   152 GΩ None@3 kV  6 3.75    52 GΩ None@3 kV  7 4   3.3 GΩ None@3kV  8 4.25   236 MΩ None@3 kV  9 4.5    79 MΩ None@3 kV 10 5    33 MΩNone@3 kV 11 10   136 kΩ None@3 kV

The dielectric composition of each of the Examples and ComparativeExperiments was printed in 0.75″ (1.9 cm) square pad 1 on a glass slide2 as shown in FIG. 1. The dielectric composition was then fired in airat 525° C. A test pattern consisting of 15 bars 3, 20 mil (0.51 mm) widewith 20 mil (0.51 mm) spaces between bars was screen printed over thedielectric composition. The bars consisted of silver paste screenprinted and then fired at 525° C. in air and emitter paste issubsequently screen printed on top of the silver lines and fired at 510°C. in nitrogen. The silver paste was a composition 7095 available fromE. I. du Pont de Nemours and Company, Wilmington, Del. The emitter pastefor these Examples was prepared by mixing three components: one asuspension containing single wall carbon nanotubes, one a typicalorganic medium containing 10% ethylcellulose and 90% beta-terpineol, andone a typical paste containing silver. Laser ablation grown single wallcarbon nanotubes were obtained from Tubes @ Rice, Rice University,Houston, Tex. as an unpurified powder produced by laser ablation. Ananotube suspension was prepared by sonicating, i.e. by mixingultrasonically, a mixture containing 1% by weight of the nanotube powderand 99% by weight of trimethylbenzene. The ultrasonic mixer used was aDukane Model 92196 with a ¼ inch horn operating at 40 kHz and watts. Theemitter paste was prepared by combining the nanotube suspension/organicmedium/silver pastes in the ratios by weight of 27/40/33. Thecombination was mixed in a three-roll mill for ten passes to form theemitter paste.

Alternate bars are connected electrically as indicated 4. A resistancevalue for the dielectric composition can be determined with anelectrometer 5 for this particular line pattern. The data show that theconductive oxide starts to take effect for this configuration when theconcentration in the paste is ˜3% and the resistance decreases to ˜200GΩ. Resistances for all the Examples and Comparative Experiments areshown in Table V.

Field emission tests were carried out for all the Examples andComparative Experiments using a flat-plate emission measurement unitcomprised of two electrodes, one serving as the anode or collector andthe other serving as the cathode. The cathode consists of a copper blockmounted in a polytetrafluoroethylene (PTFE) holder. The copper block isrecessed in a 1 inch by 1 inch (2.5 cm×2.5 cm) area of PTFE and thesample substrate is mounted to the copper block with electrical contactbeing made between the copper block and the sample substrate by means ofcopper tape. A high voltage lead is attached to the copper block. Theanode is held parallel to the sample at a distance, which can be varied,but once chosen it was held fixed for a given set of measurements on asample. A spacing of 1.25 mm was used. The anode consists of a glassplate coated with indium tin oxide deposited by chemical vapordeposition. It is then coated with a standard ZnS-based phosphor,Phosphor P-31, Type 139 obtained from Electronic Space ProductsInternational. An electrode is attached to the indium tin oxide coating.The test apparatus is inserted into a vacuum system, and the system wasevacuated to a base pressure below 1×10⁻⁵ torr (1.3×10⁻³ Pa). A negativevoltage pulse with typical pulse width of 3 μsec at a frequency of 60 Hzcan applied to the cathode or a constant voltage can be applied. Theimage emitted by the phosphor as a result of the emission current isrecorded with a camera.

The charging problem during field emission and the significantimprovement with the dielectric compositions of this invention can bedemonstrated using these samples. As a control, the emitter cell ofComparative Experiment A which contains no SnO2:Sb filler was studiedwith both pulsed and constant anode voltage. FIG. 2 a shows the diodeemission image when high voltage pulses of 3 μsec duration were appliedacross the anode-cathode gap at 60 Hz. The onset of charging inducedemission, which can be seen as bright patches in FIG. 2 a, was observedat 2 kV. The bright patches spread rapidly with time and the charginginduced emission became uncontrollable within a few seconds of itsonset. When a continuous anode voltage was applied, charging inducedemission occurred at an anode voltage of 1 kV as shown in FIG. 2 b.FIGS. 3 a and 3 b demonstrate the effectiveness of using a dielectriccomposition of this invention to eliminate charging during fieldemission. When the same diode emission tests were conducted with theemitter cell of Example 5 which contains 3.5% SnO2:Sb, no charginginduced emission was observed at pulse anode voltage of 3 kV andcontinuous anode voltage of 1.5 kV as shown in FIGS. 3 a and 3 b,respectively.

1. A dielectric composition comprising (a) a mixture of oxides that isfireable in air at a temperature in the range of about 450° C. to about550° C., and (b) antimony-doped tin oxide in an amount of from about 0.5wt % to about 15 wt % of the total weight of the composition wherein themixture of oxides (a) comprises a solid solution comprised of about 1 toabout 26 wt % SiO₂, about 0.5 to about 6 wt % Al₂O₃, about 6 to about 24wt % B₂O₃, about 2 to about 24 wt % ZnO, about 0.1 to about 5 wt % Na₂Oand about 20 to about 75 wt % Bi₂O₃.
 2. The dielectric composition ofclaim 1 which has been fired in air at a temperature in the range ofabout 450° C. to about 550° C. and is subsequently fired in nitrogen ata temperature in the range of about 450° C. to about 550° C.
 3. Thedielectric composition of claim 1 wherein the mixture of oxides (a)comprises a solid solution comprised of about 1 to about 9 wt % SiO₂,about 0.6 to about 6 wt % Al₂O₃, about 6 to about 14 w % B₂O₃, about 2to about 13 wt % ZnO, about 0.1 to about 2 wt % Na₂O and about 65 toabout 72 wt % Bi₂O₃.
 4. The dielectric composition of claim 1 furthercomprising one or more of a photoinitiator, a developable binder and aphotohardenable monomer.
 5. The dielectric composition of claim 1 in theform of a screen-printable paste.
 6. The paste of claim 5 wherein saidpaste can be photopatterned.
 7. A dielectric that comprises acomposition according to claim
 1. 8. An electron field emission devicethat comprises a dielectric according to claim
 7. 9. A multi-layerdielectric wherein the top layer comprises a composition according toclaim
 1. 10. An electron field emission device that comprises adielectric according to claim
 9. 11. A dielectric composition comprising(a) a mixture of oxides that is fireable in air at a temperature in therange of about 450° C. to about 550° C., and (b) antimony-doped tinoxide in an amount of from about 0.25 wt % to 25 wt % of the totalweight of the composition; wherein the mixture of oxides (a) comprises asolid solution comprised of about 1 to about 26 wt % SiO₂, about 0.5 toabout 6 wt % Al₂O₃, about 6 to about 24 w % B₂O₃, about 2 to about 24 wt% ZnO, about 0.1 to about 5 wt % Na₂O and about 20 to about 75 wt %Bi₂O₃.
 12. The dielectric composition of claim 11 which has been firedin air at a temperature in the range of about 450° C. to about 550° C.and is subsequently fired in nitrogen at a temperature in the range ofabout 450° C. to about 550° C.
 13. The dielectric composition of claim11 which comprises antimony-doped tin oxide in an amount of from about0.5 wt % to about 15 wt % of the total weight of the composition. 14.The dielectric composition of claim 11 wherein the mixture of oxides (a)comprises a solid solution comprised of about 1 to about 9 wt % SiO₂,about 0.6 to about 6 wt % Al₂O₃, about 6 to about 14 w % B₂O₃, about 2to about 13 wt % ZnO, about 0.1 to about 2 wt % Na₂O and about 65 toabout 72 wt % Bi₂O₃.
 15. The dielectric composition of claim 11 in theform of a screen-printable paste.
 16. The paste of claim 15 wherein saidpaste can be photopatterned.
 17. A dielectric that comprises acomposition according to claim
 11. 18. An electron field emission devicethat comprises a dielectric according to claim
 17. 19. A multi-layerdielectric wherein the top layer comprises a composition according toclaim
 11. 20. An electron field emission device that comprises adielectric according to claim
 19. 21. An electron field emission devicethat comprises an anode and a cathode; wherein the cathode comprises aconductive layer, an emitter layer and a dielectric layer; wherein theemitter layer comprises a carbon nanotube emitting material; wherein thedielectric layer comprises a dielectric material comprising (a) amixture of oxides that is fireable in air at a temperature in the rangeof about 450° C. to about 550° C., and (b) antimony-doped tin oxide inan amount of from about 0.25 wt % to 25 wt % of the total weight of thematerial; wherein the emitter layer is arrayed at lines or dots ofemitting material, and dielectric material in the dielectric layer ispositioned adjacent to the lines or dots of emitting material.
 22. Thedevice of claim 21 wherein the dielectric layer has been fired in air ata temperature in the range of about 450° C. to about 550° C. and issubsequently fired in nitrogen at a temperature in the range of about450° C. to about 550° C.
 23. The device of claim 21 wherein the amountof antimony-doped tin oxide present in the dielectric material is fromabout 0.5 wt % to about 15 wt % of the total weight of the composition.24. The device of claim 21 wherein the dielectric material comprises asolid solution comprised of about 1 to about 26 wt % SiO₂, about 0.5 toabout 6 wt % Al₂O₃, about 6 to about 24 w % B₂O₃, about 2 to about 24 wt% ZnO, about 0.1 to about 5 wt % Na₂O and about 20 to about 75 wt %Bi₂O₃.
 25. The device of claim 21 wherein the dielectric materialcomprises a solid solution comprised of about 1 to about 9 wt % SiO₂,about 0.6 to about 6 wt % Al₂O₃, about 6 to about 14 w % B₂O₃, about 2to about 13 wt % ZnO, about 0.1 to about 2 wt % Na₂O and about 65 toabout 72 wt % Bi₂O₃.
 26. The device of claim 21 wherein the dielectriclayer comprises a multi-layer dielectric wherein the top layer comprisesa dielectric material comprising (a) a mixture of oxides that isfireable in air at a temperature in the range of about 450° C. to about550° C., and (b) antimony-doped tin oxide in an amount of from about0.25 wt % to 25 wt % of the total weight of the material.